1. Field of the Invention
This invention relates generally to rendering systems and more particularly to a method and apparatus for generating a dispersed cluster screen that produces stable light-tones and minimizes visual repeated patterns.
2. Description of the Related Art
Printers are now a commonplace item in the home as well as the office environment. Through the process of halftoning, a bi-level device, e.g., a laser printer, transforms an image with greater amplitude resolution to an image with lesser amplitude resolution. The basic premise is to trade off resolution for greater apparent tone depth. Digital halftoning techniques may be classified as frequency modulation (FM) screening or amplitude modulation (AM) screening. FM screening, also known as stochastic screening, uses algorithms that place dots in neither completely regular nor completely random fashion. One of the biggest advantages of FM screening is that it eliminates moiré patterns that can form in halftones. AM dot patterns, e.g., clustered dot screen and dispersed dot screen, are ordered schemes as they share a common grid or cell structure. An AM dot pattern breaks up an image into dots of varying sizes to simulate the original image. FM screening, on the other hand, keeps the dots the same size and varies the frequency, or number, of dots and the location of those dots to simulate the original image.
The printing mechanism for a laser printer tends to be unstable when printing a single dot. Dispersed dot screen techniques require stable formation of an isolated single dot; therefore, such techniques are more suitable for display screens and not for a laser printer. FIG. 1A illustrates a dispersed dot screen. Block 102 is the original basic halftone array. Base image unit 100 is an 8×8 block that repeats, i.e., an equivalent halftone block. As can be seen in block 104, isolated single dots occur, thereby causing instability in printing device such as a laser printer. Also, the regular structure of the screen can be seen.
Clustered dot screen techniques cluster dots together for a stronger signal. However, the structures of clustered dot screens tend to be visible due to the periodicity. Furthermore, moiré patterns tend to appear with the application of clustered dot screens for color printing as two or more periodic structures are superimposed. FIG. 1B illustrates a clustered dot screen. Block 106 is the original basic halftone array. Block 108 is its rectangular equivalent block. As can be seen in block 110, the structure of the clustered dots is readily visible.
FIG. 1C represents a stochastic screening model, i.e., an FM technique. A shortcoming of the FM technique with bi level devices, such as a laser printer, is that for light tone regions and mid tone regions single isolated dots may be required to be printed, as illustrated in block 112. Thus, the laser printing mechanism becomes unstable for the light and mid tone regions and does not satisfactorily reproduce the small halftone dots associated with the stochastic screening model.
As a result, there is a need to solve the problems of the prior art to provide a technique that substantially eliminates any visible patterns and is stable for light and mid tone regions when printing an image.